This invention relates to surveying instruments and more particularly to a spirit level for easy attachment to the sight rod used by surveyors to ensure their true vertical, or plumb, position.
On May 10, 1994, U.S. Pat. No. 5,309,644 issued to the applicant and a co-inventor. In that patent apparatus for measuring sewer invert elevation is disclosed. This apparatus includes a conventional surveyor's sight rod which must be held in a true vertical, or plumb, position during measurement. A circular spirit level is clamped on the rod for this purpose, employing a sleeve with a set screw for the purpose. To ensure proper positioning of the level at right angles to the rod, the clamping screw must be tightened securely. This may result in marring the rod surface, thereby making it difficult to slide the level off and onto the rod. If the level is not at the optimal height for viewing, the screw must be released before the level can be readjusted, then tightened at the desired position.
U.S. Pat. No. 4,343,093 issued Aug. 10, 1982 to Eadens discloses a circular spirit level with a pair of right angle arms for engaging a rectangular object to be plumbed, with a spring biased bead chain for wrapping about the object for attachment thereto.
U.S. Pat. No. 4,785,544 issued Nov. 22, 1988 to Heinsius et al. discloses a spirit level held to a drill by a clamp with a gap.
U.S. Pat. No. 4,908,949 issued Mar. 20, 1990 to Jaccard discloses a similar clamp with a gap.
These would not satisfy the precision requirements of the surveyor. U.S. Pat. No. 4,879,816 issued Nov. 14, 1989 to Sierk teaches a spirit level intended for attachment to a surveyer's sight rod by attachment at the screw joint between segments of the rod. It is not easily slipped on and off and cannot be adjusted to any convenient vertical position between joints.